Health and economic benefits of meeting WHO air quality guidelines, Western Pacific Region

Abstract Objective To quantify the number of avoidable annual deaths and associated economic benefits from meeting the World Health Organization (WHO) air quality guidelines for ambient concentrations for fine particulate matter (PM2.5) for Member States of the WHO Western Pacific Region. Methods Using the AirQ+ software, we performed a quantitative health impact assessment comparing country-level PM2.5 concentrations with the 2005 and 2021 air quality guidelines recommended maximum concentrations of 10 and 5 μg/m3, respectively. We obtained PM2.5 data from the WHO Global Health Observatory (latest available year 2016), and population and mortality estimates from the United Nations World Population Prospects database for the latest 5-year period available (2015–2019), which we averaged to 1-year estimates. A risk estimate for all-cause mortality, based on a meta-analysis, was embedded within AirQ+ software. Our economic assessment used World Bank value of a statistical life adjusted to country-specific gross domestic product (latest available year 2014). Findings Data were complete for 21 of 27 Member States. If these countries achieved the 2021 guidelines for PM2.5, an estimated 3.1 million deaths would be avoided annually, which are 0.4 million more deaths avoided than meeting the 2005 guidelines. China would avoid the most deaths per 100 000 population (303 deaths) and Brunei Darussalam the least (5 deaths). The annual economic benefit per capita ranged from 5781 United States dollars (US$) in Singapore to US$ 143 in Solomon Islands. Conclusion Implementing effective measures to reduce PM2.5 emissions would save a substantial number of lives and money across the Region.


Introduction
Air pollution is a major determinant of health and causes annually an estimated 6.7 million premature deaths worldwide. Only high blood pressure, tobacco use and dietary risks surpass air pollution as causing more premature deaths. 1 Evidence suggests that fine particulate matter (particles ≤ 2.5 µm in diameter, PM 2.5 ) is the main air pollution component causing harm, contributing annually to an estimated 4.1 million premature deaths worldwide. 2 Accordingly, PM 2.5 is one of the most monitored, regulated and studied air pollutants. 2 Some of the highest ambient concentrations of PM 2.5 are recorded in Member States within the World Health Organization (WHO) Western Pacific Region. While this Region only hosts one quarter of the world's population, approximately one third of the global PM 2.5 -attributable deaths happen here. 1 A systematic review of cohort studies conducted in the Region and a worldwide meta-analysis support the association between long-term exposure to PM 2.5 and increased all-cause mortality. 3,4 While PM 2.5 concentrations are decreasing in some areas of the Western Pacific Region, 5 the increases in major sources of PM 2.,5 , such as traffic, industry, energy production and agriculture, are putting populations at higher risk from ambient compared to indoor levels of PM 2.5 . 6 Thus, assessing outdoor PM 2.5 concentrations' impact on public health is important to protect populations. Some sources of PM 2.5 are easier to manage than others. Localized sources may be addressed at the community or country levels, such as by implementing lower-emission transport policies: motorized traffic is a major contributor to localized PM 2.5 levels in South-East Asian and Oceanian countries. 7 However, transboundary air pollution is a major source of PM 2.5 in countries such as Republic of Korea, 8 with naturallysourced dust a major contributor in northern China, 7 requiring coordinated regional action with neighbouring countries.
To minimize harm, WHO has developed global air quality guidelines to support policy-makers. The guidelines focus on particulate matter, ozone, nitrogen dioxide and sulfur dioxide. 9,10 In 2021, WHO updated the 2005 guidelines 10 using evidence collected in the preceding 15 years. WHO realized that much more ambitious targets for ambient air pollution concentrations were needed to minimize harm, 9 and in the updated guidelines the annual ambient concentration target for PM 2.5 was halved from 10 to 5 µg/m 3 . This target is an ideal scenario that policy-makers may want to achieve through regulations.
Tools and measures are available for decision-makers to successfully manage air pollution through policy implementation and action. For example, a report from the United Nations (UN) Environment Programme provides 25 science-based clean air measures for policy-makers in Asia and the Pacific. 11 The WHO-developed AirQ+ software supports health professionals and institutions to quantify the effects of ambient Health, economic benefits and air quality, Western Pacific Region Nicole Egerstrom et al.
air pollution exposure through health impact assessments. 12 Furthermore, policy-makers may consider the value of a statistical life, that is, an economic value used to quantify the benefit of avoiding a death, as a critical parameter in economic analyses for benefits and costs of regulatory actions. 13 Here, we aimed to estimate the impact of Western Pacific Region's Member States achieving the WHO air quality guidelines for PM 2.5 annual ambient concentrations, 9,10 by quantifying the number of avoided deaths and related economic benefit.

Data
Detailed description of data used in the health impact assessment is available in the online repository. 14 We obtained the latest populationweighted country estimates of ambient PM 2.5 concentrations (year 2016) from the indicator metadata registry list in WHO's Global Health Observatory. 15 We used combined total estimates for residence area types (rural and urban), as separate estimates were not available for many countries and the in-built AirQ+ functionality currently only accommodates country-wide estimates. As such, we assumed that all subregions within each country have the same concentrations for this specific year.
For population and mortality data, we focused on individuals 30 years or older because concentration-response functions are limited to this higher age range; individuals younger than 30 years old are not expected to die from diseases caused by air pollution. We obtained 2015-2019 population and mortality estimates for each country from the UN World Population Prospects. 16 A 1-year average of this 5-year period was calculated for all countries with available data; assuming that year-to-year fluctuations occur, thus an average providing a more accurate temporal estimate.

Concentration-response function
Our analysis used all-cause mortality attributable to air pollution as the outcome for ease of comparison across countries, in absolute and population-adjusted (per 100 000 population) terms. We calculated the estimated PM 2.5 -attributable proportional mortality as the estimated number of attributable cases divided by the population per 100 000 and the mortality rate per 100 000 people.
We performed a quantitative health impact assessment using AirQ+ version 2.1.1 (WHO European Region, Copenhagen, Denmark), 17 which has in-built concentration-response functions derived from published scientific literature. In addition to the all-cause mortality concentration-response function, the software provides responses for mortality from specific causes, including lung cancer, chronic obstructive pulmonary disease, ischaemic heart disease and stroke, among adults. We decided to focus on mortality from all causes combined for simplicity. Within AirQ+ software, we used the embedded standard relative risk (RR) of the concentration-response function for all-cause mortality at a given level of outdoor PM 2.5 concentration, that is RR: 1.08 (95% confidence interval, CI: 1.06-1.09). This RR is from a global meta-analysis of 104 studies. 4 We adapted the above-listed data for input to AirQ+, and visualized AirQ+ health impact assessment results using R version 4.0.5 (R Foundation, Vienna, Austria) 18 in RStudio version 1.4.1106 (Posit, Boston, United States of America), 19 with packages dplyr, ggplot2, readr, readxl and tidyr.
As the factual for the health impact assessments, we used the annual estimates of country-level ambient PM 2.5 concentration. As counterfactuals for the health impact assessments, we used the 2021 and 2005 annual ambient air quality guidelines PM 2.5 target concentration values of 5 and 10 µg/m 3 , respectively. We used obtained population and mortality data to evaluate the standing of countries towards meeting the WHO air quality guidelines.

Economic assessment
To demonstrate the economic benefit of meeting the 2021 WHO air quality guidelines among the Western Pacific Region Member States, we used the value of a statistical life per Member State. 13 Briefly, the value of a statistical life per Member State was calculated and published previously with adjustment by each country's 2015 World Bank income classification for gross domestic product in 2014. 13 The published values of a statistical life were calculated on a base value of a statistical life of 9.6 million United States dollars (US$) (in 2014), a sum which is based on United States labour market estimates. We calculated the absolute economic benefit per country as the estimated number of attributable deaths to PM 2.5 concentrations above the air quality guidelines multiplied by the value of a statistical life per country in US$ millions. For easier interpretation and comparison across countries, we calculated values per capita per country by dividing the absolute economic value per country by population of that country. The value of a statistical life was not available for all countries, thus we only conducted an economic assessment for those countries with available data.

Results
Estimates for mortality and population were available for 21 of 27 Member States; Cook Islands, Marshall Islands, Nauru, Niue, Palau and Tuvalu lacked data. Only Cook Islands and Niue had no available value of a statistical life data. We therefore excluded these six countries from our analysis; however, as small island states, we expect them to contribute with a very small percentage of the total regional values for population, deaths and economic benefit. We  Table 1).

Health impact assessment
The estimated number of attributable deaths from annual exposure to ambient PM 2.5 concentrations above the 2005 air quality guidelines (10 µg/m 3 ) or the 2021 air quality guidelines (5 µg/m 3 ) is shown in Fig. 1 Table 3).

Discussion
In the Western Pacific Region, millions of lives and trillions of dollars could be saved annually by achieving the 2021 WHO air quality guidelines for PM 2.5 . While the number of avoided deaths varied greatly by country, we observed the greatest number, even when adjusting for population size, in China, followed by Mongolia and Viet Nam, which may be attributed to these countries having the highest observed PM 2.5 concentrations. While some countries with lower PM 2.5 concentration levels and/or small populations would avoid fewer total deaths (e.g. Australia, Singapore and New Zealand), the higher values of a statistical life in these countries made the associated economic benefits higher. The substantial heterogeneity seen in value of a statistical life estimates across countries may be attributable to differences in income levels, life expectancies, age distributions and social norms regarding risk and death. 20 According to latest available data, three countries have achieved the 2005 guidelines for the ambient annual PM 2.5 concentration, while none have achieved the 2021 target. However, it should be noted that the data used in this study were collected in 2016, years before the 2021 target became available. The current evidence base suggests that there is no safe concentration threshold, and improvements to air quality below the current air quality guidelines are expected to bring further benefits. 9 Our estimated number of avoidable deaths from ambient PM 2.5 in the Western Pacific Region is approximately three quarters of the 4.2 million deaths worldwide estimated by the 2019 Global Burden of Disease Study. 2 The economic benefit is approximately equivalent to the 2020 gross domestic product (GDP) of Japan, and one third of China's 2020 GDP, two of the largest economies in the Region. 21 For six countries, all small island states, we were unable to identify population and mortality data; thus, we had to exclude these countries from the analysis. Mortality and/or population Health, economic benefits and air quality, Western Pacific Region Nicole Egerstrom et al.
data may be difficult to obtain for small island states due to resource constraints. This issue raises the important question as to whether countries can also allocate resources for interventions necessary to achieve air quality guidelines, despite the economic benefits. While we did not perform a cost-benefit analysis, studies using such an analysis have shown net benefits for action on air pollution, on their own 22 or in combination with climate change mitigation actions. 23 The 25 clean air measures for Asia and the Pacific report, based on high-quality data and state-of-the-art modelling, suggests that if actions are properly implemented in Asia and the Pacific, it may positively affect the health of 1 billion people at a cost of only 5% of the projected annual GDP increase. 11 Therefore, countries might be able to achieve the 2021 air quality guidelines without enduring large economic burden to reduce air pollution levels.
We have used a simple example to demonstrate the usefulness of a health impact assessment and monetization of the avoidable deaths. Policy-makers may want to expand on our example through time trends, such as conducting the same health impact assessment for 2005-2009, to compare to our study period of 2015-2019 and demonstrate whether countries are on track towards meeting air quality guidelines. Policymakers may advance on our methods and concepts by obtaining more granular information to help with air pollution control abatement measures, for example, using gridded global concentration (alongside population) data to estimate differences in health impacts between rural and urban areas, if ground-level monitoring does not yet exist. A recent review of studies within the Asia-Pacific region concluded that methods of assessing exposure (e.g. ground-level monitoring) should be improved. 3 Nevertheless, it may be useful to assess emissions inventories per country, sector or source, if available (or modelled inventory data if not), to advocate for air pollution mitigation measures targeting specific sectors and sources. For example, policy-makers may refer to the Climate Trace independent greenhouse gas emissions tracking database. 24 Considering other air pollutants, such as nitrogen dioxide and ozone, may be useful to obtain a more complete estimate of the overall health burden due to air pollution. However, to do such analysis, robust concentration-response functions for these pollutants must be available. Our results could be built upon by performing further analyses using the AirQ+ embedded exposureresponse functions for outdoor air pollution, which also consider household air pollution and second-hand smoke exposures. These health impact assessments could consider cause-specific mortality, including chronic obstructive pulmonary disease, lung cancer, stroke and ischaemic heart disease. In AirQ+, these cause-specific mortality rates are based on the 2019 Global Burden of Disease Study. 25 However, cause-specific mortality rates for Western Pacific Region Member States are not yet available from the UN data source that we used; an alternative non-UN source for these data may be the Global Health Data Exchange. 26 Policy-makers could also consider country-specific health impact assessments using local rather than global concentration-response functions for all-cause mortality; 27 however, there may not yet be an adequate Health, economic benefits and air quality, Western Pacific Region Nicole Egerstrom et al.
number of local studies on outdoor PM 2.5 pollution and health for conducting a meaningful meta-analysis. While deaths are the main driver of economic benefits because of the high value of a statistical life, investigating years of life lost and value of a statistical life year (i.e. the economic value of illness preceding death, but not death itself) might be useful for having a more complete picture of the economic benefits of achieving air quality guidelines; however, the cost of illness is much lower than the cost of death. 25 By design, the different counterfactual scenarios in health impact assessments support decision-makers to identify which regulations best meet their economic and industry criteria for future strategic goals. The 25 clean air measures for Asia and the Pacific report details 25 specific policy actions that can be directly implemented within the Western Pacific Region to decrease PM 2.5 exposure levels and thus improve public health. 11 The report groups these policy actions into three different categories, one of which focuses specifically on reducing conventional emissions that lead to PM 2.5 formation. With the Western Pacific Region Member States expected to grow their economies collectively by approximately 80% by 2030, the report focuses on strengthening compliance from industry and energy-related emission sources (e.g. power plants and transport). For example, the regulation of the transport sector to reduce diesel emissions from heavy vehicles, which is a major source of PM 2.5 , could greatly improve public health. Focusing on emissions controls and air pollution prevention programmes, as the 25 clean air measures for Asia and the Pacific report states, along with the findings in this study on economic and health impacts, could support policy-makers to make more effective changes in air quality for improved population health.
Our study has some limitations. First, the concentration-response functions are based on published studies mainly from Europe and North America and might not be generalizable to Western Pacific Region Member States. Second, we used PM 2.5 data averaged for the whole country, and did not distinguish between rural and urban areas. Some cohort studies in the Asia-Pacific region (namely China and Republic of Korea) have suggested different health effects from long-term exposure to PM 2.5 between rural and urban areas of residence. 28,29 Such differences may in part be due to different sources of PM 2.5 such as greater household air pollution among rural households. 3 Not being able to consider subregional variability within Member States, due to a lack of data and in-built functionality of AirQ+, hinders us in our study of the differences among attributable deaths within urban versus rural areas of residence. This variability may prove to be important given that some of the main sources of PM 2.5 come from urban (e.g. commercial, industrial and transport) activities. Future work may create subregional health impact assessments within AirQ+, or perform the health impact assessment outside of AirQ+ using more sophisticated software, such as geographic information Research Health, economic benefits and air quality, Western Pacific Region Nicole Egerstrom et al.
systems. Finally, we did not differentiate between gender, however women may experience higher risk than men due to additional PM 2.5 exposures from household air pollution when performing gender normative roles, 30 compounding health inequality. 31 WHO-supported AirQ+ software is an adequate advocacy tool for exercises in risk communication. The United States Environmental Protection Agency has compared their BenMAP tool to AirQ+ and found the results to be comparable. 32 However, AirQ+ is more user-friendly than BenMAP and thus more likely to be used by policymakers. AirQ+ users can customize the concentration-response functions available in AirQ+, allowing the user to evaluate cause-specific or sourcespecific deaths. We annualized 5-year aggregated data, which provides the advantage of smoothing out year-to-year variation in mortality and population data. This advantage is important for countries with smaller populations in which estimated attributable deaths could be sensitive to outliers.
Here we show that implementing effective policy and regulations to reduce PM 2.5 emissions and exposure by meeting the WHO air quality guidelines could avoid deaths and produce an economic benefit for the Western Pacific Region. Member States may require assistance to better monitor ambient air pollution levels, collect mortality and population data and obtain local GDP-adjusted values of a statistical life to improve decision-making for mitigative actions.
Note: The 2005 WHO air quality guidelines for annual PM 2.5 concentration is a maximum of 10 µg/m 3 , 10 and for the 2021 guidelines the maximum concentration is 5 µg/m 3 . 9 We estimated the number of avoidable deaths using the AirQ+ tool. 17 ■ Competing interests: None declared.